1. Field of the Invention
The present invention relates to an integral type dielectric filter which removes noises or adjacent channel signals from a signal stream in a portable communication apparatus. Particularly, the present invention relates to an integral type dielectric filter in which the insertion loss is minimized, and the damping characteristics desired by the user are satisfied.
2. Description of the Prior Art
Recently, various portable communication apparatuses are flooded. In the midst of this flooding, users want portable communication apparatuses which do not just have communication functions, but also are cheaper and more miniaturized so as to make it more convenient to carry with them. To cater to this trend, manufacturers are producing portable communication apparatuses of small volume and light weight.
As a conventional dielectric filter, there is U.S. Pat. No. 5,537,082 as illustrated in FIG. 1a. In this filter, two through holes 101 and 102 are formed side by side in a dielectric block, and a conductive electrode is formed on the entire surface of the dielectric block. On an upper face 300 which is parallel with the through holes 101 and 102, there are formed input/output electrodes 301 and 302. The conductive electrode of the first face 100 is removed, and a non-metallic coupling region 201 is formed on a second face 200 (which is lying opposite to the first face 100) between the through holes 101 and 102 by regionally removing the conductive electrode. Thus a capacitance coupling is formed between the through holes 101 and 102 which serve as resonators.
This dielectric filter is a capacitance coupling filter which has pass characteristics over the low frequency bands. In this dielectric filter, if a damping characteristic of -30 dB or less is to be satisfied at a low frequency band (about 903 MHz) as desired by users, the pass band shows a damping of -4.0 dB as illustrated in FIG. 1b, with the result that the insertion loss is aggravated.
Japanese Patent Laid-open Gazette No. Hei-10-126107 discloses another structure of the dielectric filter as shown in FIG. 2a. In this dielectric filter, a conductive electrode is formed on the entire surface of the dielectric block in which through holes 101 and 102 are formed. Further, non-metallic regions 101a and 102a are formed respectively within the through holes 101 and 102 by removing the conductive materials, thereby forming an electrical coupling between the through holes 101 and 102.
In this dielectric filter, the insertion loss at the mean frequency (about 927 MHz) is -2.5 dB, and thus its insertion loss is lower than that of the dielectric filter of FIG. 1a. At the damping point (about 903 MHz or about 949 MHz), however, a maximum damping characteristic of -19 dB is seen. Thus, if a satisfactory damping characteristic of -30 dB or less is to be obtained, the insertion loss is aggravated up to -4.0 dB.
Further, conventionally in order to satisfy the damping characteristics, the insertion loss has to be allowed to be increased as described above. However, in view of the price competitions between the manufacturers and the limitation of the frequency resources, there is a demand for a dielectric filter which is accompanied by a low manufacturing cost and shows a superior damping characteristic.